I'm sorry, but that's not an elephant and it's certainly not in the room!

Even in the US that concept barely works (they end up "giving away" many of the seats in "First" - though anecdotally from briefly browsing the American carriers' forums here it seems that these upgrades are getting harder to come by lately) - and there's simply no real demand for a similar offering in Europe (it's one thing to say that you want that kind of separate cabin - but a different thing entirely to actually pay for the increased fares that that would entail).

By giving up the greater degree of flexibility (both the Y and J cabins can be dynamically re-sized - so a "shortfall" in J bookings can still allow an "overflow" of Y bookings), the cost of each J seat would surely be far far greater than it currently is, having sacrificed flexibility and efficiency for a separate J cabin.

And even if you decide only to introduce that sort of separate J cabin for some premium-heavy or longer-distance medium haul routes - you lose efficiency because you now have a separate sub-fleet which can only be used on these routes. But the feweer different aircraft types you have in your flieet, the easier it is to utilise your entire fleet more effectively and efficiently (or - you disappoint your high-paying J customers expecting an "extra wide reclining seat" when a mechanical issue means they have to insted travel on a "bog-standard" plane configured with the movable curtain and the free middle seat for the J cabin, instead, because that's the only aircraft type available)

If that style of separate, differentiated J cabin would work in Europe - one of the very very many airlines operating here (the market is still more fragmented than the US) would have introduced it by now and made a killing.

Just because something is the norm, or the standard, or seems to work in the US - which is a very very different market - does not mean it would work in Europe - even if of course the average passenger would of course like a "bigger" seat.